1997 GMC Sonoma Owner's Manual - Page 266

Radiator Pressure Cap Thermostat Engine coolant temperature is controlled by a thermostat in the engine coolant system. The thermostat stops the flow of coolant through the radiator until the coolant reaches a preset temperature. Power Steering Fluid NOTICE: Your radiator capis a 15 psi (105 m a ) pressure-type cap and must be tightly installed to prevent coolant loss and possible engine damage from overheating.Be sure the arrows on the cap line up with the overflow tube on the radiator filler neck. 6-28